Studies will be made of the conduction of propagated action potentials in canine cardiac Purkinje fibers in which the normal Na- dependent action potential has been abolished. Such studies will include the study of the electrical activity of fibers exposed to Na- free, Ca-rich solutions. The effects of antiarrhythmic drugs on such electrical activity will be examined with a view to seeing whether such drugs suppress either slow conduction or spontaneous activity in such fibers. Efforts will be made to develop other preparations that show propagated action potentials under circumstances in which the normal action potential is abolished. The overall goal will be to relate the above studies to the mechanism of action of clinically effective antiarrhythmic agents.